On the op mal frac ona on scheme for lymphocyte infiltra on in GBM radiotherapy treatment

Lorea Iturri Marjorie Juchaux cristele Gilbert Julie espenon Yolanda Prezado ^*

• INSERM U1021 Signalisation Normale et Pathologique de l'embryon aux Thérapies Innovantes des Cancers, Orsay, France

Some radioresistant and immunosuppressive tumors, such as glioblastoma mul forme (GBM), are s ll a challenge and current clinical prac ce, surgical resec on and chemoradia on, do not yet offer effec ve treatments. Immunotherapy (IT) has emerged as a powerful tool in the arsenal against cancer. IT was also explored in GBM phase III clinical trials, with unsuccessful results since it is cri cally dependent on the availability of preexis ng an -tumor immunity. Given its immunomodulatory poten al, RT could be employed as a tool to inflame tumors for increasing their response to IT. The radia on configura on to achieve the cri cal tumor inflamma on needed for IT success remains elusive. In this study the op mum dose frac ona on scheme to maximize immune cell tumor infiltra on was assessed.Two orthotopic rat glioma models, having different vasculariza on and immunogenicity levels, were irradiated with three different dose frac ona on schemes. Tumor immune cell popula ons were assessed by flow cytometry.One single high dose (25 Gy) or extreme hypofrac ona on is needed to elicit a significant immune infiltra on in the tumors.